1. Field of the Invention
This invention relates to constant printing pressure mechanisms for a label printing and dispensing machine, particularly of the hand-held type (hereinafter referred to as a "label printing machine"). These machines are usually of the type in which each label piece of a pressure sensitive self-adhesive label strip is imprinted during the motion of the action levers of the machine.
2. Description of the Prior Art
In conventional constant printing pressure mechanisms, an inking means is urged by spring members toward the type faces that are disposed at the lower part of a printing head. The printing head is attached to printing levers. There is also a hand operated lever. The printing levers and the hand lever are usually separated formed from each other.
In these label printing machines, however, the force of the printing stroke against the label varies directly with the force of squeezing of the hand lever, so that the depths or darknesses of printed figures are liable to correspondingly vary. In order to eliminate this disadvantage, constant printing pressure mechanisms have been developed in recent years.
In some constant printing pressure mechanisms, use is made of the compressive action of a constant pressure spring that is provided between the hand lever and the printing levers. It is only the squeezing of the hand lever that increases the pressure of the constant pressure spring against the printing levers. When the spring pressure exceeds the control limit of the constant pressure mechanism, printing is initiated.
In another variation of such a mechanism, an inking device is rotatably mounted at the front portion of the label printing machine and it has a return spring that brings the inking device into contact with the type faces of a printing head. In this mechanism, when the hand lever is squeezed, the printing head is depressed and this moves the inking device aside against the force of a return spring. Only when the downward force of the printing levers that are pressed by a compression spring exceeds the control limit of the return spring, is the label printing performed. Accordingly, the printing stroke is effected only by the compression spring, and label printing at a constant pressure can be attained, regardless of the intensity of the squeezing of the hand lever. Printed figures of even darkness can be obtained.
The foregoing constant pressure mechanisms only use springs in the printing levers. The force of these springs is liable to become weaker through repeated use. This weakens the force of the printing strokes. As a result, the conventional constant printing pressure mechanisms do not print labels with constant depth of printed characters.
Further, in conventional structures, high precision printing of labels cannot be attained since double printing is caused by the rebounding or bouncing of the printing head on the platen.
Furthermore, conventional printing mechanisms print for an unpredictable time period dependent upon the duration of the squeezing of the hand lever. When the hand lever is squeezed for too long a time, the type faces of the printing head are held too long in contact with the label then on the platen. Excess ink is absorbed by the label surface, which results in blurring of the ink.
It has become necessary in recent years to read information printed on labels optically as the P.O.S. (point of sales) system has become widely adopted. Therefore, labels must be printed quite precisely in order to be readable by optical character reading machines. For this purpose, it has become necessary to provide the label printing machine with two or more inking rollers so as to apply ink evenly to the type faces of a printing head, and/or with guide means for guiding the inking rollers to roll over the type faces under a constant pressure.
The structure of label printing machines becomes complicated with known constant printing pressure mechanisms and the label printing machines become expensive for general use.